Metal detectors are popularly used by hobbyists and collectors to search for buried or otherwise hidden metal objects of value or particular interest, such as coins, jewelry, and artifacts of historical significance. The metal detectors typically employ a transmit coil through which current flows, the current establishing time-varying magnetic fields that induce eddy currents in metal objects, and interact with any magnetic permeability of the metal object. These effects are detected in a receive coil, and are used to identify the metal objects.
In addition to metal objects of interest, the ground itself typically contains metallic compounds, particularly compounds containing iron. The iron compounds in particular have a relatively high magnetic permeability that often masks the response of the detector to the metal objects in the ground. It is a problem in the metal detector art to eliminate all the ferrous mineral signals in the target volume of ground while retaining sufficient information to identify the metal objects.
The ground also may be primarily electrically conductive as opposed to magnetically permeable or ferritic. For example, beach sand contains salt water. For this type of ground signals detected by the receive coil may be indicative of the conductivity of the ground rather than the metal objects. For either ferritic or conductive ground, it is necessary to minimize or eliminate the effect of the ground on the received signals, and this is a problem in the art.
It may be noted that, traditionally, “induction balance,” “frequency domain” or “continuous wave” metal detectors employ a sinusoidal interrogating signal at a specific frequency. However, even where the interrogating signal is not sinusoidal, the signal is often Fourier analyzed into specific frequency components. The response therefore has a magnitude and phase that are in general altered from the magnitude and phase of the original signal as a result of the (complex) impedance of the target.
The impedance of the target has a real part that produces a (vector) component of the response that is in-phase with the interrogating frequency, and a so-called imaginary part that produces another component of the response that is in-quadrature, or 90 degrees out of phase with the interrogation frequency. It may be noted that the real component of a vector is often identified in engineering and mathematics with the horizontal “x” axis of a standard Cartesian coordinate system, while the imaginary component is identified with the vertical “y” axis. However, in the metal detector art, metal detector responses are sometimes graphed so that the real part of the response is plotted on a vertical “Y” axis that represents zero phase shift, with a horizontal “X” axis depicting negative and positive phase deviations from the “Y” axis. This scheme is used herein.
The real, or “Y” component of a given frequency specific response vector represents the effect of the conductivity of all of the material contributing to the response at the specific frequency, while the imaginary, or “X” component represents the effect at the frequency of the reactance of this material. For non-ferrous metal materials and salt-water, the resistive component of the response will be much greater than the reactive component; conversely, for ferrous metals and soil containing iron, the reactive component is larger than the resistive component. For a given, highly conductive non-ferrous object at a suitably high frequency, e.g., a silver dollar at 50 kHz, nearly all of the response will fall on the “X” axis. The “Y” component of the response may be negligible. The ratio of the “X” component to the “Y” component depends on the material of which the object is formed, the size and shape of the object, and the interrogating frequency.
The detector resolves the total frequency specific response into its Y and X components, each providing information about the target volume of ground. In frequency domain metal detectors, that information is typically the phase shift of the response with respect to the transmitted signal.
In addition to the requirement to distinguish the response from ground, it is necessary to be able to distinguish one metal object from another, i.e., to distinguish one phase shift from another. Accomplishing both of these objectives generally requires two independent methods for detecting responses that are insensitive to ground.
In one of these methods, termed “motion discrimination,” the user is required to keep the transmit coil in motion. The ground is not necessarily homogeneous, but both the permeability and the conductivity of the ground are relatively constant over a limited area, so that detected signals resulting from ground are relatively slowly varying compared to signals resulting from the much more localized metal objects; the metal objects cause changes in the detected signals that are perceived by the metal detector to have a relatively high frequency. Then, high or band-pass filtering may be used to filter out the low frequency portion of the detected signals corresponding to the ground.
In the other conventional method for detecting responses that are insensitive to ground, often referred to as “ground balancing,” a location on the ground is selected for calibrating the detector, and it is determined how to linearly combine the X and Y components of the response so that it is zero, or put another way, it is determined how much to rotate the X and Y coordinate system to align it with the phase angle of the ground so that, at the phase angle of the ground, the response is nulled. In practice, this can be achieved by varying the phase angles of respective synchronous demodulators so that the demodulators are insensitive to components with a phase equal to the phase angle of the ground.
More recently, metal detectors have been provided that employ two interrogating frequencies, so that four response components may be obtained as described above. A lower frequency is provided that is particularly suited for detecting larger objects, especially those of good conductors like copper or silver, and a higher frequency is provided that is more suited for detecting smaller objects and objects that are composed of metals which are relatively poor conductors. The user of such a detector may select between the two frequencies depending on the type of object that the user is searching for.
To ground balance dual frequency detectors, Candy, U.S. Pat. No. 4,942,360, proposes forming various linear combinations of the four X and Y components. For example, to null the response for reactive soil, the '360 Patent proposes among other things forming a linear combination of the reactive components for the two frequencies. The use of two interrogation frequencies is apparently to provide for ground balancing both salt water and ferritic ground. Motion discrimination would still be required to provide an independent means of measuring phase shift independent of the ground.
Other attempts at dual frequency discrimination have not been successful. For example, one method in use uses a ratio of the Y terms to identify targets. However, these signals are not free of the effects of ground which severely compromises the performance of the method. A similar method has been used to rotate the Y axes as needed to balance the ground, and the ratio of the rotated values of Y is used to identify targets. However, a significant error is introduced by the rotation. In addition, it is a problem in all of these methods that ferrous and non-ferrous metals can be confused with one another, and it is often desired to be able to discern that a metal object is ferrous so that the user can decide to expend no further effort to uncover the object.
Motion discrimination has the obvious disadvantage that it is demanding of the user in terms of both effort and skill. It may also result in ambiguities where there are multiple metal objects in the target volume. Accordingly, there is a need for a metal detector employing static discrimination that permits discriminating between metal objects and ground, and between ferrous and non-ferrous metal objects without requiring that the search coil of the metal detector be in motion.